Document Type : Research Article
Authors
Department of Food Science and Technology, College of Agriculture, Isfahan University of Technology, Isfahan, Iran.
Abstract
Introduction: Flavor, taste, odor and color of fresh apple juice is unstable during concentration process and storage. Pectic substances and phenolic compounds are responsible for these physicochemical changes. So, decreasing of these compounds is necessary to improve the appearance and marketability of apple juice. In fruit juice industry, clarification is one of the most important steps which removes pectin, polyphenols and other unfavorable components and prevents haze formation during storage. Enzymatic clarification (depectinization) can be applied for removal of pectic substances by using pectinase. This enzyme can hydrolyze pectin and produce pectin-protein complexes which would be settled easily. Also in depectinization step, some enzymes such as amylase and amyloglucosidase can be used to degrade starch. Fining agents such as gelatin, bentonite, activated carbon, silicasol or a combination of these compounds can be used in conventional clarification. Sepiolite is a natural fibrous phyllasillicate clay mineral with a formula Mg8Si12O30(OH)4(OH2)4. nH2O(n=6-8) which has specific physicochemical properties such as high porosity, surface area and adsorption capacity. This clay clay has been used in several important applications such as decolorization of sugar juice, removal of phosphorus from vegetable oil and enhancing decoloration of crude palm oil. In this research, efficiency of Sepiolite for apple juice clarification was evaluated. First, the response surface methodology (RSM) was used to optimize apple juice clarification conditions. Then, a comparison among Sepiolite and other commercial fining agents in respect to clarification efficiency was accomplished. Finally, three types of zero-, first- and second-order kinetic models were used for explanation of changes in turbidity, color, viscosity and total phenolic content (TPC) during clarification process of apple juice.
Materials and Methods: The pasteurized and unclarified apple juice, Bentonite, Gelatin (Bloom= 80-100), Amylase (Alphamyl MG) and Pectinase (Pectofruit XL) were gratefully obtained from Behnoosh Iran Co. (Shahrekord, Iran). Sepiolite (with specific surface area of 105 m2/g) was purchased from Farapooyan Isatis Yazd Co. (Yazd, Iran). The kieselgel and chemicals with analytical grade were purchased from Merck Co. (Germany). Sepiolite was activated with hydrochloric acid according to Balci’s method with slight modification. For evaluation of Sepiolite changes, some analysis like pH, acidity, density and the moisture content of Sepiolite clay were done before and after the activation by the method of Sabah and Çelik. Specific surface area was measured by using the BET method (Belsorp mini II, Japan). XRF method (PW148, Philips Co.) was used for determining of difference between Sepiolite elemental compositions before and after the activation. A pretreatment was conducted to improve the performance of fining agents according to Türkyilmaz et al.’s method with slight modification. The swelling capability of fining agents in water increases their clarification capacity. In the method of RSM, the independent factors at three levels were concentration of activated Sepiolite (0.05-0.1% w/v), temperature of clarification (50-60˚C) and process time (2-10 h). Juice turbidity was regarded as response. As the first step in clarification of apple juice, amylase and pectinase enzymes were applied (25 µl of each enzyme per 100 mL juice at 20˚C). The mixture was stirred and heated in an incubator at 55˚C for 1 h. Sepiolite was mixed with juice in certain concentrations, stirred and placed in definite temperature and time according to each run of RSM. Finally, the juice was centrifuged (12000 rpm for 5 min) to remove the clay from clarified juice. For kinetic studies, the above steps were done in the optimized conditions using 0.05% fining agents at 50˚C for 7 h. A portable turbidometer (MARTINI, Mi 415, Romania) and the capillary viscometer (Ubbelohde-Viscometer, Fisher, USA) were used for measuring the juice turbidity and viscosity at 20˚C and expressed as NTU (Nephelometeric Turbidity Unit) and centipoise (cp), respectively. Color was measured at 20˚C by using a color meter (ZE 6000, Nippon DENSHOKU). The parameters which used for color expression were L* and a* parameters based on CIE Lab system. The Folin-Ciocalteu reagent was used for measuring the total phenolic content of juice and expressed as mg gallic acid equivalent per 100 mL juice. To evaluate Sepiolite performance in juice clarification, following fining agents were applied at optimal conditions: sepiolite (S), commercial bentonite (B), and combination of these agents with gelatin (G) and kieselgel (K) (S+G, B+G, S+G+K, B+G+K). The used concentrations of bentonite, gelatin and kieselgel were 0.05%, 0.015% and 0.04% (w/v), respectively. Sampling was conducted at 1 h intervals to evaluate the changes in juice turbidity, viscosity, color and TPC during clarification process. The rates of changes were determined by three types of zero-, first- and second-order kinetic models.
Results and Discussion: Results showed that the activated Sepiolite had less amounts of weight loss, density and pH than the native sample. On the other hand, moisture content and acidity increased. Also, acid activated Sepiolite had higher amounts of SiO2 groups. The second-order polynomial (quadratic) model was suggested as the best for describing the optimum conditions of clarification with insignificant lack of fit and high R2 (0.9845). Based on the results, process time had a significant (p
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